enable serializers for single value rollups.
Do not include a stat unless it is valid.
toString() methods for some rollup classes that have no impls.
reader writer fixes to support preaggregation:

* reader can fetch all metadata values
* writer includes metrics_locator for generic metrics operations.
integration test for `MetricsWriter`

includes a class that sends things into the system (`DoesStuff.java`)
utility class used to get data out of the system.